Filter element

ABSTRACT

A filter element comprises a pleated metal fiber fleece being pleated according to pleating lines providing an edge with pleat openings to be closed in order to make gas flowing through the metal fiber fleece. The filter element further comprises at least two flanks, each of these flanks comprising a stiff material layer. The stiff material layer and the metal fiber fleece are connected using a layer of ceramic adhesive, which prevents direct contact of the metal fiber fleece over the length of the edge of the metal fiber fleece with the flank. Meanwhile the flanks, being essentially perpendicular to the pleating lines, closing the pleat openings in order to prevent bypasses.

FIELD OF THE INVENTION

The present invention relates to filter elements, which may beregenerated electrically. More specific, the invention relates to filterelements for filtering diesel exhaust gasses.

BACKGROUND OF THE INVENTION

Diesel soot particulate traps comprising pleated metal fiber fleece areknown, e.g. from U.S. Pat. No. 5,709,722.

Diesel soot particulate traps, which can be regenerated via electricalheating of the filter element itself, are known, e.g. from U.S. Pat. No.5,800,790.

The presently known filter elements, suitable for electricalregeneration, have the disadvantage that most of the thermal energy,obtained by Joule effects out of electrical energy and used to heat thefilter element, is lost due to thermal losses.

SUMMARY OF THE INVENTION

It was found that the losses of thermal energy is caused by 3 effects:

-   1. The filter medium, generating the thermal energy via Joule    effects, looses thermal energy via radiation, e.g. towards the    filter housing.-   2. Thermal energy is lost via convection, heating the gasses which    pas through the filter medium during regeneration. This effect is    much larger when the strip is regenerated in stream.-   3. Thermal energy is lost due to thermal conduction. E.g. when the    filter medium is welded to the housing, a lot of thermal energy is    transferred from the filter medium to the housing via this contact.    The housing is needlessly heated by this thermal energy conducting.

It is an object of the invention to provide a filter element, to beregenerated electrically, which has a reduced thermal energy loss.Further, it is an object of the present invention to improve the contactbetween filter medium, being electrically regeneratable, and the housingof the filter element.

It is also an object of the invention to provide a filter unit,comprising at least two, but possibly more than two filter elements,each filter element being regeneratable individually. Such a filter unitas subject of the invention may be used in a diesel exhaust filter packfor stationary diesel engines of for diesel engines, used in vehiclessuch as boats, trains or other motor vehicle.

Filter pack is to be understood as a filter system, which is installedor used in a gas stream. It comprises a gas inlet, a gas outlet, and atleast one filter unit, installed between inlet and outlet.

A filter element as subject of the invention comprises a pleated metalfiber fleece. This metal fiber fleece, preferably sintered, is pleatedaccording to pleating lines, so providing an edge with pleat openings.The gas, to be filtered, has to flow from one side of the fleece (inflowside) to the other side of the fleece (outflow side), passing throughthe fleece. Appropriate pleat openings have to be closed in order tomake the gas to flow through the metal fiber fleece so preventingbypasses from gas from the inflow side to the outflow side, withoutpassing through the metal fiber fleece.

A filter element according to the invention further comprises a filterelement housing, which comprises at least two flanks.

According to the present invention, each of these flanks comprising astiff material layer. A relatively thick layer of ceramic adhesive isused to connect the metal fiber fleece and a stiff material layer,preferably a metal plate or rim to each other. The adhesive is at leastpresent over the whole length of the edge of the metal fiber fleece, butpreferably, the whole surface of the side of the stiff material layer iscoated with this adhesive. The metal fiber fleece is mounted between theflanks having its pleating lines preferably essentially perpendicular tothe stiff material layer of the flanks. The layer of ceramic adhesive isto prevent direct contact over the total length of the edge of the metalfiber fleece, being connected to the flank. It positions the metal fiberfleece, provides the electrically and thermally insulating propertiesand offers a good seal between the metal fiber fleece and the stiffmaterial layer. A thermally and electrically insulating side is soprovided to the stiff material layer. The metal fiber fleece is mountedbetween the thermally and electrically insulated sides of both flanksprovided by the adhesive. The flanks exercise a clamping force on theedges of the metal fiber fleece in a direction essentially parallel tothe pleating lines. Meanwhile these flanks close the pleat openings inorder to prevent bypasses.

In order to improve the adhesion between stiff material layer andceramic adhesive, a wire mesh, an expanded or perforated metal sheet maybe inserted between the surface of the stiff material layer and the edgeof the metal fiber fleece. This mesh or expanded or perforated metalsheet acts so to say as anchoring points for the ceramic adhesive, andit is sunken in the adhesive layer. Best results were obtained using ametal rim and a metal mesh. The metal mesh was spot welded to the metalrim on several points.

The thickness of the adhesive layer is preferably more than 0.5 mm, andless than 2 mm. The edge of the metal fiber fleece is sunken over acertain depth in the adhesive layer, providing a so-called sunken partto the edge of the metal fiber fleece. This sunken part has a height ofpreferably at least 10% less than the thickness of the adhesive layer,but also preferably in the range of 0.5 mm to 2 mm.

Further, the adhesion between stiff material layer and ceramic adhesivemay be obtained by first coating, e.g. spraying a layer of ceramicparticles (e.g. by flame spraying of Al₂O₃ or SiO₂) on the side of thestiff material layer, before providing the ceramic adhesive to the stiffmaterial layer. This layer also further improves the electricalinsulation between metal fiber fleece and stiff material layer, whichmay be required in case the stiff material layer is a metal plate orrim. Such spraying may be done also on the mesh or perforated orexpanded metal sheet. Possibly a ceramic layer is sprayed on the mesh orexpanded or perforated sheet, after it has been spot welded to e.g. ametal rim or plate.

To further improve the ductility and the resistance to thermal cyclingof the ceramic adhesive layer between flanks and sintered metal fiberlayer, metal particles may be added to the ceramic adhesive. Metal shortfibers are preferred over metal powder, since the ductility of curedceramic adhesive is much more superior as compared to ceramic adhesivecomprising metal powder. Surprisingly it was found that the electricalinsulation properties of such adhesive layer were influenced onlyslightly, as compared to pure ceramic adhesion.

Short metal fibers preferably comprises fibers with an equivalentdiameter “D” between 1 and 150 μm preferably between 2 and 100. Mostpreferably the diameter ranges between 2 and 50 μm or even between 2 and35 μm such as 2, 4, 6.5, 8, 12 or 22 μm. Preferably, but notnecessarily, short metal fibers have an L/D-ratio of more than 5,preferably more than 10, wherein L stands for the average length of theshort metal fibers.

Preferably, the layer of ceramic adhesive comprises at least 0.5% byweight of short metal fibers, most preferably more than 10% by weight oreven more than 20% by weight. Preferably the layer of ceramic adhesivecomprises less than 30% by weight of short metal fibers.

In general, and also according to the present invention, “stiffmaterial” is to be understood as an inflexible material, which to acertain extend lacks suppleness or pliability and being having theproperty of being difficult to bend, as is generally known for ceramicor metal plates.

According to the invention, the thermally and electrically insulatingside of the flanks closes the pleat openings, which are to be closed inorder to prevent bypasses from gas to be filtered. These sides fix themetal fiber fleece in its position.

Filter elements, as subject of the invention may further comprise otherelements, to form, together with the flanks mentioned above, the filterelement housing. These elements may also be thermally and electricallyinsulated, in order to reduce the thermal energy, lost due to radiation,from the metal fiber fleece to these elements or due to the heating ofthese elements because of contact between hot gas and housing. E.g. aperforated metal screen or a more permeable thermally insulating fabricmay be applied, in order to further reduce the thermal losses due toradiation towards the adjacent filter units of the filter pack wall. Incase of a more permeable thermally insulating fabric, preferably, aSiO₂-grid woven fabric is used.

Such filter elements as subject of the invention have severaladvantages.

The thermal energy loss due to conduction is prevented, since the sidesof the flanks, used to close the pleat openings have thermallyinsulating properties. The metal fiber fleece is only in contact withthe filter housing via this side. The pleating of the metal fiber fleecealso causes thermal radiation, being radiated from one pleat to theadjacent pleats.

Since electrical current is to be supplied only to the metal fiberfleece, in order to regenerate the fleece, the fleece is electricallyinsulated from the filter housing at its edge, by the electricallyinsulating side.

Preferably, the metal fiber fleece is to be resistant to bulging. Asintered and pleated metal fiber fleece has a rather high bulgingresistance due to the pleated shape, to provide an edge.

In the scope of the present invention, with metal fiber fleece is meanta fleece, comprising metal fibers, preferably steel fibers. The alloy ofmetal or steel may be chosen dependant on the temperature range which isto be withstand by the metal fiber fleece. Stainless steel fibers ofAISI alloys of the 300- or 400 series, or alloys such as Inconel® are tobe preferred. In case high temperatures are to be withstand duringregeneration, alloys comprising Fe, Al and Cr are preferred, such asFecralloy®. The fibers may be obtained by any presently known productionmethod, such as bundle drawing or shaving. Fiber diameters between 1 and100 μm are to be used, preferably between 2 and 50 μm, e.g. between 12and 35 μm such as 12, 17 and 22 μm. Preferably the fleece is sinteredusing appropriate sintering circumstances, according to the alloy used.Preferably, the metal fibers are obtainable by bundle drawing or coilshaving. The latter is described more in detail in WO97/04152.

A metal plate or rim is preferably provided out of stainless steel. Mostpreferably the metal fiber of the metal fiber fleece and the metal plateor rim are out of the same metal alloy.

Also thickness, weight per m², pore diameter and other fleece parametersmay be chosen, according to the particles which are to be retainedand/or the application for which the filter element is to be used.

Preferably, the metal fiber fleece used to provide the filter elements,as subject of the invention comprises different layers of metal fibers.Each fiber layer comprises fibers with a certain equivalent diameter.Best filtering results were obtained when a layer with the coarsestfibers is facing the inflow side of the filter element, whereas a layerof metal fibers with the finest fibers is facing the out-flow side ofthe filter. An example of such layered metal fiber fleece is a metalfiber fleece comprising a layer of metal fibers with equivalent diameterof 35 μm, and a layer of metal fibers with an equivalent diameter of 17μm. Possibly a layer of metal fibers with equivalent diameter of 22 μmcan be located between these two layers. Porosity of more than 85% ispreferred, while the weight per square meter of the fleece is preferablyless than 1500 g/m², e.g. 1450 g/m².

Equivalent diameter is to be understood as the diameter of a radial cutof an imaginary round fiber, having an identical surface as the radialcut of the fiber under consideration.

According to the present invention, preferably the metal fiber fleececonsists of only one strip of filter media comprising metal fibers. Mostpreferably, this strip is rectangular. However alternatively, the metalfiber fleece may consist of more than one strip of filter mediacomprising metal fibers which strips are mounted between the two flanksof the filter element as subject of the invention.

Sintered metal fiber fleece has a good resistance against buckling, whenput under mechanical load in a direction, parallel to the plane surfaceof the fleece. To improve the buckling resistance, the fleece may becorrugated using preferably repetitive undulations, with a wavelengthpreferably less than 5 times the thickness of th fleece. The amplitudeof the corrugation is also preferably less than 5 times the thickness ofthe fleece. The buckling resistance may be improved more than 50% inambient circumstances. Then the fleece is heated to more than 600° C.,the buckling improvement is still more than 30%.

The metal fiber fleece, used to provide a filter element as subject ofthe invention further comprise at least two but possibly more than twocontact bodies, fixed, e.g. clamped on or sintered to the metal fiberfleece. According to the present invention, a contact body is a body towhich the electric current is supplied by the electric circuit, in orderto regenerate the filter element. This contact body divides in a properway the electric current over the total surface of the metal fiberfleece. Preferably, these contact bodies are metal foils, e.g. Ni-foilor metal woven meshes, sintered at both ends of the metal fiber fleece.

Special care is to be taken in case the metal fiber fleece is pleated insuch a way that both ends of the metal fiber fleece, each of them to becontacting one pole of the electric circuit, are located close to eachother. Both contact bodies are to be insulated from each other. This canbe done by inserting one or more electrically insulating plates betweenboth contact bodies, e.g. mica plates. Both contact bodies may beconnected to this electrically insulating plate using bolts and nuts oralike. Preferably, the contact bodies are applied on the ends in such away that the contact bodies extend from the metal fiber fleece in theoff-stream direction of the filter element.

An alternative improved filter element may also be obtained in an otherway. Each flank may comprise a ceramic plate, which comes into contactwith the edge. The metal fiber fleece is clamped between those twoceramic plates. The ceramic plate provides thermal and electricalinsulating properties to the flanks. Possibly these ceramic plates haverecesses. The recess is preferably larger than 0.5 mm, but may be in therange of 0.5 mm to 2 mm. These recesses are obtainable by providing e.g.a slot in the thermally and electrically insulating ceramic plates.These slots correspond with the edge, in such a way that they engageclosely with the edge when the pleated metal fiber fleece is mountedbetween the two flanks. The edge is introduced in the slots, soproviding a recess of the edge. The slot fits perfect to the edge.

It should be noted that the edge is installed in the slot in such a waythat small movements, e.g. thermal expansions or vibrations, of thepleated metal fiber fleece may be allowed. This freedom of movement isobtained by providing slots, which are slightly deeper than the recessof the edge in the thermally and electrically insulating side. Lesspreferably, although possibly, the metal fiber fleece is glued to theceramic plate using ceramic or high temperature resistant adhesive.

To, protect the ceramic plates against mechanical damages, the ceramicplates may be supported by a metal plate, being present at the otherside of the ceramic plate, not contacting the metal fiber fleece.Alternatively, this metal plate may have the shape of a rim, in whichthe ceramic plate fits.

Alternatively, the ceramic plate of such a flank, according to theinvention, may be replaced by a thermally and electrically insulatingfabric, e.g. a ceramic textile layer, which is supported by a stiffmaterial layer, preferably a metal plate or rim.

A recess is also obtainable by such flanks, which comprise a stiffmaterial layer, and a thermally and electrically insulating fabric on atthe thermally and electrically insulating side of the flank. The edge ofthe pleated metal fiber fleece is then squeezed or pressed between thethermally and electrically insulating flanks. Since the pleated metalfiber fleece has sufficient buckling resistance, the pleated metal fiberfleece is pressed into the thermally and electrically insulating fabric,a recess of the edge of the pleated metal fiber fleece in the thermallyand electrically insulating sides of the flanks is so provided.

The recess of the edge should at least be sufficient to prevent thepleated metal fiber fleece to move along with the gas to be filtered.This phenomenon is so called ‘blow through’. The recess is preferablylarger than 0.5 mm, but may be in the range of 0.5 mm to 2 mm.

In the scope of the present invention, a thermally and electricallyinsulating fabric is to be understood as a nonwoven, woven, braided orknitted textile fabric, comprise thermally and electrically insulatingfibers at the surface of the fabric, which is to contact the edge of themetal fiber fleece. Most preferably, the whole fabric consist of suchthermally and electrically insulating fibers, however, a combination ofthermally and electrically insulating fibers at the side contacting theedge, with metal fibers at the opposite side may be used. Such thermallyand electrically insulating fibers preferably are ceramic fibers, suchas fibers, comprising Al₂O₃ and/or SiO₂, e.g. NEXTEL®-fibers.

The fabric thickness is preferably between 3 to 6 mm. A woven ornonwoven fabric is preferred.

When according to the invention, flanks comprising slots are used toprovide a filter element as subject of the invention, the thermally andelectrically insulating side of the flanks are obtainable by usingceramic materials, e.g. based on Al₂O₃ and or SiO₂ or mica to providethis side of the flank. The flank may be provided out of one material,or may comprise different layers, provided by different materials. Oneunderstands that, in case different layers are used to provide theflanks, the slots are to be provided in layers, which are thermally andelectrically insulating.

Filter elements as subject of the invention may further comprise otherflanks, to form, together with the flanks mentioned above, the filterhousing, providing an inlet and an outlet to the filter element. Theseflanks may also be thermally and electrically insulated, in order toreduce the thermal energy, lost due to radiation, from the metal fiberfleece to these plates or due to the heating of the housing of thefilter element due to contact between hot gas and housing.

Further, surprisingly it was found that, when a filter element assubject of the invention comprising a thermally and electricallyinsulating fabric is used, e.g. to filter diesel exhaust gas, loadedwith soot particles, the filter element works self-sealing, even afterregenerating. This is explained as follows.

The edge of the metal fiber fleece is mounted or pressed between thethermally and electrically insulating sides of the flanks.

In case a thermally and electrically insulating fabric is used, due tothe textile nature of the fabric, the metal fiber fleece is recessed toa certain depth in the fabric. Under normal circumstances, this recessis sufficient to close all voids in the fabric next to the recessed partof the metal fiber fleece, so no gas can bypass the metal fiber fleecethrough the thermal and electrical insulating fabric. In case there is asmall void in the fabric, which is not closed by the recessed part ofthe metal fiber fleece, small amounts of exhaust gas will bypass themetal fiber fleece via this void. The soot, being present in the exhaustgas, will be trapped by the fabric, so closing the void space. When themetal fiber fleece is now regenerated, the thermal and electricalinsulating fabric will not be heated enough in order to incinerate thesoot, trapped by the fabric at the void space. So the bypass of gasthrough the fabric is hindered after the void spaces are filled withsoot, due to such bypass. Th filter seals itself.

An identical effect is obtained when the edge of the pleated metal fiberfleece is mounted in a slot in the thermally and electrically insulatingside of a flank. As said, a small void space is provided underneath theedge, to allow small movements. The slot fits that good to the edge atthe surface to the pleated metal fiber fleece, that under normalcircumstances, no gas can bypass the metal fiber fleece via the sides ofthe edge and these voids. In case there is a small gap between the sideof the edge and the slot, soot will be trapped and retained in thesegaps. When the filter is regenerated, the soot will not be heated enoughin order to incinerate. So the bypass of gas through the gaps ishindered after the gaps are filled with soot, due to such bypass. Thefilter seals itself.

Filter elements as subjects of the invention are used to provide filterunits. Several filter elements may be combined, e.g. stacked one on topof the other. To avoid thermal losses, the different filter elements areseparated from each other by a thermally insulating layer, e.g. athermally insulating and thermal resistant layer of textile, e.g. awoven SiO₂-fabric.

Filter elements as subject of the invention may be used to filter hotgases, such an exhaust gases from diesel internal combustion engines.Several filter elements or filter units comprising filter elements assubject of the invention may be used in parallel, e.g. to be able toregenerate at least one filter element, through which no gas flows, soreducing convection heat losses, while the other filter elementscontinue to filter the gas stream. They may be mounted in seriesconnection, to filter the gas stream in different steps, e.g. fordifferent particle sizes.

Each filter element can be regenerated individually, preferably oneafter the other. The filter element may be regenerated inline, while gascontinues to flow through the filter element, or off-line, while gas ispartially or fully prevented to flow through the filter element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIG. 1 shows schematically a general view of a filter unit as subject ofthe invention

FIG. 2 shows schematically an enlarged view of part AA′ of the filterunit of FIG. 1.

FIG. 3 shows schematically a section according to the plane BB′ of thefilter unit of FIG. 1.

FIG. 4 shows schematically a side view of the contact bodies from afilter element as subject of the invention.

FIG. 5 shows schematically a view of alternative contact bodies from afilter element as subject of the invention.

FIG. 6, FIG. 7 and FIG. 8 show schematically a section according to theplane BB′ of an alternative embodiment of a filter unit as subject ofthe invention.

FIG. 9 shows a diesel exhaust cleaning system in a muffler-like shape,comprising different filter units as subject of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A preferred filter unit as subject of the invention is shown in FIGS. 1,2 and 3.

The filter unit comprises a number of filter elements 11, which arestacked one on top of the other. They all have a ring-like shape. Aperforated metal tube 12 is positioned inside the inner opening 13 ofthe filter element. Between each filter element, a disc-like SiO₂ feltmaterial 14 is positioned to thermally insulate the different filterelements from each other. At both ends of the filter unit, a metal plate15 is fixed against the upper and lower filter element e.g. as shown inFIG. 1 by means of a screw 16, which pushes the plate towards the filterelement. Between this plate 15 and the upper or lower filter element,another disc-like SiO₂ felt material 14 is positioned.

When this filter unit is used, preferably the gas to be filtered flowsin from the outer side of the filter elements (indicated with arrow 17),through the filter medium 18 through the perforations of the metal tube12, to the further exhaust system as indicated with arrow 19.

Taking each filter element of the present embodiment into consideration,a metal fiber fleece is used as filter medium 18. The ‘dirty’ gas flowsin via the inflow side 20, through the metal fiber fleece, via theoutflow side 21 of the metal fiber fleece to the exhaust system. Themetal fiber fleece is connected via two contact bodies 22 and 23 to anelectric circuit 24, providing electrical current to the metal fiberfleece in order to regenerate the dirt, e.g. soot, trapped in and on thefilter medium. The metal fiber fleece is preferably pleated in such away that the thermal radiation heat, generated by the pleats 25 duringregeneration, radiates to the adjacent pleats, as indicated by arrows26. An important reduction of electrical power is obtained using thisradiation heat to propagate and support the combustion of the filteredparticles

The set-up of a preferred embodiment of the filter element is shown inFIG. 2. A flank 28 of the filter element comprises a metal rim 29, towhich a wire mesh 30 is spot welded on several spots 31. A fine layer ofceramic material Al2O3 32 was sprayed on the electrical and thermalinsulating side 33 of the flank. A relatively thick layer of ceramicadhesive 34 was applied on this mesh and the electrical and thermalinsulating side 33, before the metal fiber fleece 18 was adhered to thisceramic adhesive 34, which comprises more than 10% of weight of shortmetal fibers.

The thickness of the adhesive layer was 2 mm and an adhesive based onZrO2-MgO compound was used. The edge of the metal fiber fleece wassunken in the adhesive layer over a depth of 1.5 mm. Metal plate andmetal mesh were provided out of stainless steel AISI 304. Alternativelystainless steel AISI 430 was used.

To improve the resistance to the mechanical tension, due to the fixationof the different elements on top of each other by screw 16, severalstuds 35 may be welded to the upper and lower rim of each filterelement. As shown in FIG. 1 and FIG. 2, around the filter element 11, aperforated metal plate 39 may be present (as only shown partially in theFigures for the sake of clarity).

Turning now to the contact bodies 22 and 23 of the preferred embodimentas shown in FIG. 4 and FIG. 5, a fine Ni-sheet 36 was sintered to theends of the metal fiber fleece. Both contact bodies were broughttogether and fixed to an insulating plate 37, e.g. a micaplate by meansof two bolts 38 and 39. In order to avoid electrical contact betweencontact body 22 and bolt 38, and between contact body 23 and bolt 39,two mica sheets were inserted between the insulating plate 37 and thecontact bodies 22 and 23.

An alternative set-up is shown in FIG. 5. An identical set-up as in FIG.4 is used, but the contact body 22 is shaped in such a way that nomaterial of this contact body 22 is present at behind bolt 38, fixingthe contact body 23 to the insulating plate 37. Identically, the contactbody 23 is shaped in such a way that no material of this contact body 23is present at behind bolt 39, fixing the contact body 22 to theinsulating plate 37. Using such contact bodies, the use of two micaplates 40 may be avoided, which may simplify the construction of thefilter element.

An alternative cut according to BB′ is shown in FIG. 6. The perforatedtube in this embodiment has an elliptic section. Also here, the metalfiber fleece is pleated according to pleating lines, which enablesradiation from one pleat to another during regeneration.

An other alternative cross section of a filter element as subject of theinvention is shown in FIG. 7. The filter element in this embodimentcomprises two metal fiber fleece strips, which together form the wholefilter media of the filter element. Both metal fiber fleece strips havetwo contact bodies (22 and 23), at one end each, which are connected toan appropriate electric circuit 24.

Another alternative cross section of a filter element as subject of theinvention is shown in FIG. 8. The filter element comprises a set ofmetal fiber fleece strips, each being pleated over one pleating line 81.All strips are mounted side by side. Each metal fiber fleece strip hastwo contact bodies (22 and 23), one at each end of the strip. Thecontact bodies are lined up and connected to an appropriate electriccircuit 24.

As shown in FIG. 9, gas to be filtered may enter into a muffler system,via inlet 91. Several filter units 92, each comprising several filterelements 93 are present in the muffler-like system. The gas to befiltered goes, as indicated with arrow 94, through the filter media ofeach filter element and leaves the filter unit 92 via the perforatedtube 95 in a collecting chamber 96. Via an outlet 97, the filteredexhaust gas flows further through the exhaust system as indicated witharrow 98.

As filter medium, a sintered metal fiber fleece comprising three layersof stainless steel fibers is used. A first layer comprises 600 g/m² ofFecralloy® fibers with equivalent diameter of 17 μm. A second layer ofFecralloy® fibers is applied on top of the first layer. This layercomprises 250 g/m² of fibers with equivalent diameter of 22 μm. A thirdlayer of F cralloy® fibers is applied on top of the second layer, havingfibers with equivalent diameter of 35 μm. This third layer comprises 600g/m² fibers.

A soot retention of 91% was obtained, using a stainless steel fleece,having a porosity of 85%.

The length of the metal fiber fleece in the above described embodimentsis preferably 1200 mm, while the height of the metal fiber fleece stripis preferably between 30 and 35 mm, e.g. 33.75 mm.

The soot was so-called depth filtered. This is to be understood as thefact that soot particles were trapped through the whole depth of thefilter.

Only 1 minute per element was needed to regenerate the filter unit,while consuming only 750 W to 1500 W The pressure drop over the filterelement was set to 100 mbar before regeneration.

1. A filter element comprising a pleated metal fiber fleece beingpleated according to pleating lines providing an edge with pleatopenings to be closed in order to make gas flow through said metal fiberfleece, said filter element comprising at least two flanks, each of saidflanks comprising a stiff material layer, said stiff material layer andsaid metal fiber fleece being connected using a layer of ceramicadhesive, said layer of ceramic adhesive preventing direct contact ofsaid metal fiber fleece over the length of said edge of said metal fiberfleece with a flank, said flanks being essentially perpendicular to saidpleating lines, said flanks closing said pleat openings in order toprevent bypasses, said layer of ceramic adhesive having a thickness ofmore than 0.5 mm, said thickness being less than 2 mm.
 2. A filterelement comprising a pleated metal fiber fleece being pleated accordingto pleating lines providing an edge with pleat openings to be closed inorder to make gas flow through said metal fiber fleece, said filterelement comprising at least two flanks, each of said flanks comprising astiff material layer, said stiff material layer and said metal fiberfleece being connected using a layer of ceramic adhesive, said layer ofceramic adhesive preventing direct contact of said metal fiber fleeceover the length of said edge of said metal fiber fleece with a flank,said flanks being essentially perpendicular to said pleating lines, saidflanks closing said pleat openings in order to prevent bypasses, saidceramic adhesive layer having a thickness, said layer of ceramicadhesive comprising short metal fibers having an equivalent diameter inthe range of 1 to 150 μm.
 3. A filter element as in claim 2, said layerof ceramic adhesive comprising at least 0.5% of weight of said shortmetal fibers, said layer of ceramic adhesive comprising less than 30% ofweight of said short metal fibers.
 4. A filter element comprising apleated metal fiber fleece being pleated according to pleating linesproviding an edge with pleat openings to be closed in order to make gasflow through said metal fiber fleece, said filter element comprising atleast two flanks, each of said flanks comprising a stiff material layer,said stiff material layer and said metal fiber fleece being connectedusing a layer of ceramic adhesive, said layer of ceramic adhesivepreventing direct contact of said metal fiber fleece over the length ofsaid edge of said metal fiber fleece with a flank, said flanks beingessentially perpendicular to said pleating lines, said flanks closingsaid pleat openings in order to prevent bypasses, said ceramic adhesivelayer having a thickness, said edge of said metal fiber fleece beingsunken in said layer of ceramic adhesive over a sunken part, said sunkenpart having a height at least 10% less than said thickness of said layerof ceramic adhesive.
 5. A filter element comprising a pleated metalfiber fleece being pleated according to pleating lines providing an edgewith pleat openings to be closed in order to make gas flow through saidmetal fiber fleece, said filter element comprising at least two flanks,each of said flanks comprising a stiff material layer, said stiffmaterial layer and said metal fiber fleece are connected using a layerof ceramic adhesive, said layer of ceramic adhesive preventing directcontact of said metal fiber fleece over the length of said edge of saidmetal fiber fleece with a flank, said flanks being essentiallyperpendicular to said pleating lines, said flanks closing said pleatopenings in order to prevent bypasses, said ceramic adhesive layerhaving a thickness, said flanks further comprising a wire mesh or anexpanded or perforated metal sheet.
 6. A filter element as in claim 1,said stiff material layer being a metal plate or metal rim.
 7. A filterelement as in claim 1, said layer of ceramic adhesive comprising shortmetal fibers having an equivalent diameter in the range of 1 to 150 μm.8. A filter element as in claim 7, said layer of ceramic adhesivecomprising at least 0.5% of weight of said short metal fibers, saidlayer of ceramic adhesive comprising less than 30% of weight of saidshort metal fibers.
 9. A filter element as in claim 1, said edge of saidmetal fiber fleece being sunken in said layer of ceramic adhesive over asunken part, said sunken part having a height at least 10% less thansaid thickness of said layer of ceramic adhesive.
 10. A filter elementas in claim 1, said layer of ceramic adhesive being present over thelength of said edge of said metal fiber fleece.
 11. A filter element asin claim 10, said flanks further comprising a wire mesh or an expandedor perforated metal sheet.
 12. A filter element as in claim 1, saidflanks further comprising a wire mesh or an expanded or perforated metalsheet.
 13. A filter element as in claim 1, said flanks having one sidebeing coated with a layer of ceramic adhesive, said sides making contactwith said edge of said metal fiber fleece.
 14. A filter element as inclaim 1, wherein said stiff material layer is coated with a layer ofceramic particles.
 15. A filter element as in claim 1, said filterelement comprising more than 1 strip of metal fibers.
 16. A filterelement as in claim 1, said metal fiber fleece comprising stainlesssteel fibers.
 17. A filter unit comprising at least two filter elementsas in claim 1, said filter elements being thermally insulated from eachother.
 18. A filter element as in claim 2, said stiff material layerbeing a metal plate or metal rim.
 19. A filter element as in claim 2,said edge of said metal fiber fleece being sunken in said layer ofceramic adhesive over a sunken part, said sunken part having a height atleast 10% less than said thickness of said layer of ceramic adhesive.20. A filter element as in claim 2, said layer of ceramic adhesive beingpresent over the length of said edge of said metal fiber fleece.
 21. Afilter element as in claim 20, said flanks further comprising a wiremesh or an expanded or perforated metal sheet.
 22. A filter element asin claim 2, said flanks further comprising a wire mesh or an expanded orperforated metal sheet.
 23. A filter element as in claim 2, said flankshaving one side being coated with a layer of ceramic adhesive, said sidemaking contact with said edge of said metal fiber fleece.
 24. A filterelement as in claim 2, wherein said stiff material layer is coated witha layer of ceramic particles.
 25. A filter element as in claim 2, saidfilter element comprising more than 1 strip of metal fibers.
 26. Afilter element as in claim 2, said metal fiber fleece comprisingstainless steel fibers.
 27. A filter unit comprising at least two filterelements as in claim 2, said filter elements being thermally insulatedfrom each other.
 28. A filter element as in claim 4, said stiff materiallayer being a metal plate or metal rim.
 29. A filter element as in claim4, said layer of ceramic adhesive comprising short metal fibers havingan equivalent diameter in the range of 1 to 150 μm, said layer ofceramic adhesive comprising at least 0.5% of weight of said short metalfibers, said layer of ceramic adhesive comprising less than 30% ofweight of said short metal fibers.
 30. A filter element as in claim 4,said layer of ceramic adhesive being present over the length of saidedge of said metal fiber fleece.
 31. A filter element as in claim 30,said flanks further comprising a wire mesh or an expanded or perforatedmetal sheet.
 32. A filter element as in claim 4, said flanks furthercomprising a wire mesh or an expanded or perforated metal sheet.
 33. Afilter element as in claim 4, said flanks having one side being coatedwith a layer of ceramic adhesive, said side making contact with saidedge of said metal fiber fleece.
 34. A filter element as in claim 4,wherein said stiff material layer is coated with a layer of ceramicparticles.
 35. A filter element as in claim 4, said filter elementcomprising more than 1 strip of metal fibers.
 36. A filter element as inclaim 4, said metal fiber fleece comprising stainless steel fibers. 37.A filter unit comprising at least two filter elements as in claim 4,said filter elements being thermally insulated from each other.
 38. Afilter element as in claim 5, said stiff material layer being a metalplate or metal rim.
 39. A filter element as in claim 5, said layer ofceramic adhesive comprising short metal fibers having an equivalentdiameter in the range of 1 to 150 μm, said layer of ceramic adhesivecomprising at least 0.5% of weight of said short metal fibers, saidlayer of ceramic adhesive comprising less than 30% of weight of saidshort metal fibers.
 40. A filter element as in claim 5, said layer ofceramic adhesive being present over the length of said edge of saidmetal fiber fleece.
 41. A filter element as in claim 5, said flankshaving one side being coated with a layer of ceramic adhesive, said sidemaking contact with said edge of said metal fiber fleece.
 42. A filterelement as in claim 5, wherein said stiff material layer is coated witha layer of ceramic particles.
 43. A filter element as in claim 5, saidfilter element comprising more than 1 strip of metal fibers.
 44. Afilter element as in claim 5, said metal fiber fleece comprisingstainless steel fibers.
 45. A filter unit comprising at least two filterelements as in claim 5, said filter elements being thermally insulatedfrom each other.
 46. A filter element as in claim 40, said stiffmaterial layer being a metal plate or metal rim.
 47. A filter element asin claim 40, said layer of ceramic adhesive comprising short metalfibers having an equivalent diameter in the range of 1 to 150 μm, saidlayer of ceramic adhesive comprising at least 0.5% of weight of saidshort metal fibers, said layer of ceramic adhesive comprising less than30% of weight of said short metal fibers.
 48. A filter element as inclaim 40, said flanks having one side being coated with a layer ofceramic adhesive, said side making contact with said edge of said metalfiber fleece.
 49. A filter element as in claim 40, wherein said stiffmaterial layer is coated with a layer of ceramic particles.
 50. A filterelement as in claim 40, said filter element comprising more than 1 stripof metal fibers.
 51. A filter element as in claim 40, said metal fiberfleece comprising stainless steel fibers.
 52. A filter unit comprisingat least two filter elements as in claim 40, said filter elements beingthermally insulated from each other.
 53. A method of using a filterelement for filtering diesel exhaust gas, comprising the steps of:providing a filter element as in claim 1 in an exhaust system for dieselexhaust gas; and allowing diesel exhaust gas to flow through the metalfiber fleece of the filter element for retention of soot particles outof said diesel exhaust gas.
 54. A method of using a filter element forfiltering diesel exhaust gas, comprising the steps of: providing afilter element as in claim 2 in an exhaust system for diesel exhaustgas; and allowing diesel exhaust gas to flow through the metal fiberfleece of the filter element for retention of soot particles out of saiddiesel exhaust gas.
 55. A method of using a filter element for filteringdiesel exhaust gas, comprising the steps of: providing a filter elementas in claim 4 in an exhaust system for diesel exhaust gas; and allowingdiesel exhaust gas to flow through the metal fiber fleece of the filterelement for retention of soot particles out of said diesel exhaust gas.56. A method of using a filter element for filtering diesel exhaust gas,comprising the steps of: providing a filter element as in claim 5 in anexhaust system for diesel exhaust gas; and allowing diesel exhaust gasto flow through the metal fiber fleece of the filter element forretention of soot particles out of said diesel exhaust gas.
 57. A methodof using a filter element for filtering diesel exhaust gas, comprisingthe steps of: providing a filter element as in claim 40 in an exhaustsystem for diesel exhaust gas; and allowing diesel exhaust gas to flowthrough the metal fiber fleece of the filter element for retention ofsoot particles out of said diesel exhaust gas.
 58. A method of using afilter element for filtering diesel exhaust gas, comprising the stepsof: providing a filter unit as in claim 17 in an exhaust system fordiesel exhaust gas; and allowing diesel exhaust gas to flow through themetal fiber fleece of the filter elements for retention of sootparticles out of said diesel exhaust gas.
 59. A method of using a filterelement for filtering diesel exhaust gas, comprising the steps of:providing a filter unit as in claim 27 in an exhaust system for dieselexhaust gas; and allowing diesel exhaust gas to flow through the metalfiber fleece of the filter elements for retention of soot particles outof said diesel exhaust gas.
 60. A method of using a filter element forfiltering diesel exhaust gas, comprising the steps of: providing afilter unit as in claim 37 in an exhaust system for diesel exhaust gas;and allowing diesel exhaust gas to flow through the metal fiber fleeceof the filter elements for retention of soot particles out of saiddiesel exhaust gas.
 61. A method of using a filter element for filteringdiesel exhaust gas, comprising the steps of: providing a filter unit asin claim 45 in an exhaust system for diesel exhaust gas; and allowingdiesel exhaust gas to flow through the metal fiber fleece of the filterelements for retention of soot particles out of said diesel exhaust gas.62. A method of using a filter element for filtering diesel exhaust gas,comprising the steps of: providing a filter unit as in claim 52 in anexhaust system for diesel exhaust gas; and allowing diesel exhaust gasto flow through the metal fiber fleece of the filter elements forretention of soot particles out of said diesel exhaust gas.